Glow plugs are typically associated with the cylinder chambers of Diesel engines, and are controlled by an associated electronic control module which is arranged to control in real time the amount of energy transferred to each glow plug, so as to reach and hold a predetermined working temperature. The glowing control apparatus comprises also electrical connections between a vehicle voltage supply, such as the battery of the vehicle, the glow plugs and the electronic control module. The electronic control module drives the electronic switches, generally MOSFET transistors, by means of pulse-width-modulated (PWM) control signals.
FIG. 1 is an electric diagram showing an apparatus for controlling glow plugs in a Diesel engine. In FIG. 1 reference numeral 10 generally indicates an electronic control system for driving the glow plugs GP1, GP2, GP3 and GP4 associated each with a respective cylinder chamber in a 4-cylinder Diesel internal combustion engine. The glow plugs GP1-GP4 are connected each between a respective output terminal 1-4 of the electronic control system 10 and a ground terminal EGND (“engine ground”).
In FIG. 1 a d.c. voltage supply B, such as the battery of the motor-vehicle, has its positive terminal connected to a supply input 5 of the electronic control system 10, and the negative terminal connected to a ground terminal BGND (“battery ground”). The ground terminal BGND is connected to the ground terminal EGND by a conductor 6, and is further connected to a terminal 7 of the electronic control system 10 through a conductor 8. The terminal 7 of the electronic control system is connected to an “internal ground” terminal IGND of the electronic control system 10, through a conductor 9.
The electronic control system 10 comprises four electronic switches M1-M4, having each the drain-source path connected essentially in series with a respective glow plug, between the terminals of the voltage supply B.
The electronic switches M1-M4 are, for instance, MOSFET transistors, and have their gates connected to respective outputs of a control unit 20. The control unit 20 drives said switches M1-M4 in order to realize a PWM control.
The control system 10 has a node A which is used to measure, in a known manner, the voltage across the glow plugs GP1-GP4.
The glowing control system 10 above disclosed has many disadvantages: For example: the electrical resistance of each glow plug GP1-GP4 is low, so any variation in the resistive path between the node A and the terminals 1-4 causes a variation in the voltage drop across the glow plugs, and consequently an imprecise temperature control; the glow plugs GP1-GP4 are mechanically grounded to the engine block: in fact, only the PWM control signals are supplied to the glow plugs GP1-GP4 while the electrical return path is provided by the connection between the “engine ground” terminal EGND and the “battery” terminal BGND, which provides ground return also for systems requiring high currents, like engine starter, generator, etc. . . . These high currents could cause a significant voltage drop across the conductor 6, represented as a voltage drop Vd1 on a resistor R1 of the conductor 6. Furthermore, another voltage drop Vd2 on a resistor R2 representing the resistance of the conductor 8 affects the connection between the “battery” terminal BGND and the “internal ground” terminal IGND. This means that the voltage supplied to energize the glow plugs GP1-GP4 is affected by an error due to the ground shift between the “engine ground” terminal EGND and the “internal ground” terminal IGND of the electronic control system 10, so resulting in an imprecise temperature control. These series voltage drops depend on the engine electrical architecture and the values change with the engine conditions.
The energy transferred to the glow plugs GP1-GP4 is the key variable to be controlled, and conventional glow-plug control systems generally monitor both the voltage across each glow plug and the current flowing through each glow plug. Controlling the energy transferred to the glow plugs GP1-GP4 means controlling the power transferred thereto during each period of the PWM driving signals applied to the corresponding electronic switches M1-M4. The duty-cycle of the PWM driving signals is controlled in a closed-loop, in order to supply the desired energy to each glow plug GP1-GP4.
In a first control method (voltage control) the control unit 20 defines a voltage duty factor that must be applied to each glow plug GP1-GP4. The control unit 20 performs a voltage closed loop control by monitoring the supply voltage B at the node A. The voltage duty factor is a function of said monitored voltage.
The PWM signals generated by the control unit 20 depend on the difference between the voltage at the node A and the potential at the “internal ground” terminal IGND, whereas the heating power generated in each glow plug GP1-GP4 is a function of the voltage at the node A and the potential present at the “engine ground” terminal EGND of the glow plugs GP1-GP4.
In a second control method (current control), the control unit 20 defines a current duty factor for each glow plug GP1-GP4. The control unit 20 performs a current closed loop control by monitoring the current flowing through the glow plugs GP1-GP4. The current duty factor is a function of said monitored current.
The main idea of the present invention is to identify a state variable which is not influenced by the resistive path and ground shifts between the control unit 20 and glow plugs GP1-GP4. Even if the current control method has brought good results for certain heating points, it shows low accuracies of the controlled temperature, mainly due to the electro-thermal characteristics of the components.
Furthermore, another side effect present in the control system 10 above disclosed is due to tolerances of the glow plugs: glow plug resistance can have a not negligible spread which affects the temperature.
The known voltage control minimizes the resistance spread effect on the temperature regulation, but the performances result heavily affected by the series voltage drops. The known current control rejects the series voltage drops, but the temperature regulation results heavily affected by the resistance spread effect.
It is at least one object of the present invention to provide an improved method and an improved apparatus for controlling glow plugs in a Diesel engine that includes the advantages of both a voltage loop control and a current loop control, allowing to overcome the above-outlined inconveniences of the prior art systems. In addition, other objects, desirable features, and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.